650 central's exhaust port optimizers and torque peak optimizers

[hoffman900]

Hi Jack,

I do not intend to make stepped headers for the XS. You can read my thread in the tracker section to get a sense of my background. I'm interested in package specific applications ie: a one off for a particular combination of parts. The length and steps are determined by many things. There is no, "one size fits all" - as you certainly know. Even if I was in the business, I know enough about this topic that I couldn't bring myself to sell a product that isn't optimized. A header like I'm talking about is going to work at varying degrees (good or bad) outside what it's designed for - It's only completely right for one package.

I can agree with you that a tighter bend will require a large diameter pipe. I'm not sure how velocity is recovered, you have to speed the gas up and moving into a large pipe serves to counteract that. Have you looked up literature involving 'exhaust gas blow down' ? It's interesting stuff. From what I've taken from the pros, is that the first several inches of the exhaust pipe is an extension of the port - imo, a torque cone insert greatly reduces the area creating a choke point, this at a moment where gas velocity is highest. Moving to a much larger head pipe than the port will drastically slow down this velocity at a moment in time when the exhaust valve is still open. I want to move the mass of gas out as fast as possible (per Calvin's post) while that valve is open. If I built a triple step header on my bike - it would only have a 1.5" pipe for 8-10". Keep in mind a SR has at least 500cc per cylinder compared to </= 350cc of the typical XS.

Reversion is a function of poor wave timing. Really what's going on is the negative wave is returning at the wrong point in time - this is where total exhaust length comes into play as well as steps, diameter, and megaphones. When you get these factors right, you don't need reversion devices and maintain a properly sized diameter pipe to meet gas flow requirements. The trouble is this - wave speed is dictated by temperature - the higher the temperature the faster the speed of sound. This is why EGT's are important - the wrong tune will change the pressure wave speed. Additionally, if you run alcohol, your EGT's will be different as will the mass through the pipe (more fuel is needed per unit energy than gasoline). I've asked professionals if air cooling on motorcycle exhaust is an issue in determining tune lengths, but I've been informed it is not an issue on super-comp dragsters which run through the traps at nearly 190mph, so not to worry about if for my application.

When dealing with high overlap cams, they're very sensitive to tune length. I remember a friend driving an hour to dyno test a new cam. It wouldn't run for crap and was spitting fuel out the carb the whole time. Changing the valve timing and not redesigning the exhaust caused the system to be out of sync. You can see the standoff on the dyno when dealing with cammy motors when they're 'off the cam' and the harmonic waves are out of sync.

That's my take on things, at least. It makes for a lively discussion!

Cheers,
Bob

 

[Tomterrific]

All the fellows discussing this topic have read, The Scientific Design of Intake and Exhaust Systems by Pilip Smith. This study was done back in the late 50's and continued to be revised. I suggest this book to anyone who wants a better understanding of how an exhaust and intake work together. I wrote a paper in 1967 about Gordon Blair's tuning of drag racing engines using Smith's formula. He told of a free 2 psi of supercharging with the proper setup. The lab equipment used in the tests is quite clever and well worth reading about on its own. ;^)

The above book deals heavily with acoustic tuning. Length of both the exhaust pipe and intake runner, plus the cam timing and rpm are critical. The wave formula works at a specific rpm so is great for a racing engine. At a lower rpm the waves most likely will hinder performance which is why Fueling invented the anti reversion device. You might notice some headers/pipes with a buldge somewhere along its length. This is the anti reversion devise Fueling invented. The anti reversion cone is an add on for headers/pipes that didn't use Fueling's original idea.

So, a set of anti reversion cones should not be thought of as a power adder but a device that could potentually smooth a power curve, eliminate a hole in the useful range of rpm and have an engine perform better at real life street maners. There is one good way to find out, try it.

Tom

Ps. I really like reading about results others get when they describe their setup. It's tough finding a good mix and people should sit up and take note what these guys have done. Good advice!

 

[650performance]

Assuming a given pipe is reasonably close to what might be considered an optimum design, the difference between it and the "perfect" pipe is typically very little when we're talking about maximum power ... perhaps on a few percentage points.
However, the difference can be quite stark when you look at the power curve as it tracks along with the increase in RPM.
When I was doing the dyno testing on my XS race engines in the '90s I found that there was very little difference in top end power between, say, a 1 3/4" system that was, e.g. a) 42" long, b) 50" long, or 32" with a 14" megaphone with a 1" reverse cone. They all topped out between 67 and 71 RWHP in the 7500 - 8250 RPM range.
However, the curve itself was meaningfully affected and it was a difference I could feel on the track. a) was linear but 4 HP short at the top end, b) had a "hole" in the 5500 - 6500 range and c) was linear and made the most HP.
Scientific? Conform to gas flow theory? Probably not. But, it was what worked with that engine with those components.
Toss the same pipes on another engine and the results would no doubt be somewhat different.

 

[hoffman900]

+1 to both of you guys.

What I'm calling band-aids are the ones that sit up against the port. As I said, I have one of those torque cones that came in a box - the smallest part is smaller than the exhaust valve in my motor!

The 'Hytech' type chambers and FMF 'Powerbombs' are a different story as they both occur farther along the pipe (if you notice, they are around the 8-12" range from the port face and coincide with the first step... hmm...)..

Here is the FMF patent:
http://www.patentgenius.com/patent/7510050.html

It is my understanding that a Swedish rally tuner invented these in the early '80s, but being a typical racer and not a business person, didn't patent it.

Here is a schematic of the Hytech chamber:
http://i879.photobucket.com/albums/ab351/Fe_Head/Anti_ReversionChamber.jpg

Even the merge collectors are a form of anti-reversion device.

This is a merge collector for anyone who doesn't know:
http://www.epi-eng.com/images/Engine/exhaust_technology_04.JPG

With the road race motors, they could make more peak power, but area under the curve is what we're concerned with- they have to come off the corner. Camshafts have a little less duration and a little tighter LCA than they would for drag racing (but still lots of lift), and venturi sizes are a little smaller than they would be for best peak hp.

In my mind, I would try to run the smallest pipe I could get away with to maintain velocity without negating flow, especially in the first foot or so of the pipe - get that exhaust out of there! Just like the ports in the head - both flow and velocity need to be looked at.

Of course, I would have two different sets of pipes - 1) half mile and longer tracks and 2) less than half mile. I would also like two motors; 1) a long rod screamer for the big tracks 2) a short rod torquer for the small tracks. I need to win the lottery first.

Really the idea of step header is trying to knock out some of those dips. According to Blair, a pressure wave is propagated with each sudden change in area. The steps are staged to coincide with different harmonic waves in the system. When I built my system, I assumed the start of the megaphone to be a change of area as well.

Here is the Prof. Gordon Blair methodology for the megaphone:

The Blair formula for a megaphone: (primary ID^2 × 6.25)^.5 . Angles can be from 5-9 degrees: lower = a pulse over a broader range, steeper = narrower range. Then a 14-20 degree reverse cone.

I can't find anything on outlet, but I would think about a .25-.5" larger than the last primary od should be correct.

This is a book I would like to get someday when I have the money: http://books.sae.org/book-r-186/ . Chapter 2 covers exhausts.

edit: glad to see I'm not the only one who has noticed this http://www.xs650.com/forum/showthread.php?t=11939

 

[Jack]

Hi Jack,

I do not intend to make stepped headers for the XS. You can read my thread in the tracker section to get a sense of my background. I'm interested in package specific applications ie: a one off for a particular combination of parts. The length and steps are determined by many things. There is no, "one size fits all" - as you certainly know. Even if I was in the business, I know enough about this topic that I couldn't bring myself to sell a product that isn't optimized. A header like I'm talking about is going to work at varying degrees (good or bad) outside what it's designed for - It's only completely right for one package.

I can agree with you that a tighter bend will require a large diameter pipe. I'm not sure how velocity is recovered, you have to speed the gas up and moving into a large pipe serves to counteract that. Have you looked up literature involving 'exhaust gas blow down' ? It's interesting stuff. From what I've taken from the pros, is that the first several inches of the exhaust pipe is an extension of the port - imo, a torque cone insert greatly reduces the area creating a choke point, this at a moment where gas velocity is highest. Moving to a much larger head pipe than the port will drastically slow down this velocity at a moment in time when the exhaust valve is still open. I want to move the mass of gas out as fast as possible (per Calvin's post) while that valve is open. If I built a triple step header on my bike - it would only have a 1.5" pipe for 8-10". Keep in mind a SR has at least 500cc per cylinder compared to </= 350cc of the typical XS.

Reversion is a function of poor wave timing. Really what's going on is the negative wave is returning at the wrong point in time - this is where total exhaust length comes into play as well as steps, diameter, and megaphones. When you get these factors right, you don't need reversion devices and maintain a properly sized diameter pipe to meet gas flow requirements. The trouble is this - wave speed is dictated by temperature - the higher the temperature the faster the speed of sound. This is why EGT's are important - the wrong tune will change the pressure wave speed. Additionally, if you run alcohol, your EGT's will be different as will the mass through the pipe (more fuel is needed per unit energy than gasoline). I've asked professionals if air cooling on motorcycle exhaust is an issue in determining tune lengths, but I've been informed it is not an issue on super-comp dragsters which run through the traps at nearly 190mph, so not to worry about if for my application.

When dealing with high overlap cams, they're very sensitive to tune length. I remember a friend driving an hour to dyno test a new cam. It wouldn't run for crap and was spitting fuel out the carb the whole time. Changing the valve timing and not redesigning the exhaust caused the system to be out of sync. You can see the standoff on the dyno when dealing with cammy motors when they're 'off the cam' and the harmonic waves are out of sync.

That's my take on things, at least. It makes for a lively discussion!

Cheers,
Bob

Bob, here's a link to the Diesels Yammer Hammer race bike, he's no loner involved with the XS and all the pictures have been deleted but I can tell you the headers had multiple sectional welds, look crude by all means but were functional.
http://xs650temp.proboards.com/index.cgi?action=display&board=project&thread=4783&page=1

As far as reversions goes,thats a very complex topic with multiple culprits that could be causing this, cam timing,rod ratio,port velocity,pipe diameter, cam overlap and how exhaust heat is managed to limit expansion. When you can effectively enhance the low pressure point of the exhausts pulse, scavenging efficiency increases resulting in more across the board gains while addressing reversion over any stepped header design could accomplish IMO. Stepped headers have more than often proven their inconsistency in yielding extra power during test runs than the traditional straight header. Trying to overcome atmospheric pressure at the exit during overlap is what it all boils down to.

Hope you succeed in designing the optimum exhaust for the 500

 

[scabber]

Any opinions on a twin exhaust with a crossover pipe. I have seen one with a crossover pipe about 10" from the exhaust valve. Then it had antireversion chamber below the crossover. I think it might have been on a Trtiumph.

 

[hoffman900]

Thanks Jack. Those pictures aren't working, but I can imagine what they look like. The bends on mine are a little tight for my taste on the first go around. It's fine for a street bike. When/if I get a race bike and get the head done up right, I'll build one with a 6" radius bend and make it a high pipe (much like the big radius bends you see on a top shelf XR750 Harley). That way I can run a 1.5" od pipe for about 8" without losing any flow. Stahl headers has as small as a 1.5"od pipe with a 6" radius. They're about twice as much as one on a 3-4" radius, but worth it for what I'm interested in.

Scabber,

My buddy's GS500 has a crossover like you describe. I've never seen them on anything otuside of a OEM set-up. Cars have H or X pipes, but they occur a foot or more pass the collectors. I would imagine a cross over (like the X pipe on the AMA XR1200 series bikes) or a 2-1 depends on what your crank phasing is. I don't know enough about parallel twins to say - I would imagine the stock 360 degree set up of a XS would work well with a 2-1. Not sure about 270. A twingle engine would work better as a 2-2.

All,

This is just some of the stuff I've been reading over the last ten years or so and will give you an idea of where I'm coming from.

Here is a very nicely written article which covers exhaust blowdown in the beginning. http://www.beautyofspeed.com/data/cylinder_headtech/index.htm

There is too much for me to type, but if you look up the 'port area curve' thread on www.speedtalk.com (you have to sign up to view that particular thread), you'll learn quite a bit. Here are some highlights:

Clint Gray, owner of www.tfxengine.com

With conventional headers or semi-conventional headers, pressure waves do not have a whole lot of effect on the blowdown part of the exhaust cycle - unless the header is significantly too long or too short. However, pressure waves do have a big effect on what happens during valve overlap either for good or for bad.

Unconventional header designs can use pressure waves to significantly reduce the exhaust blowdown period, thereby allowing for further reductions in exhaust valve size (increases in intake valve size) while still accomplishing the desired scavenging of exhaust residuals. Alternately if the exhaust valve size is left alone the improved blowdown can be beneficial for applications with power adders. Also the, intake valve can then be opened earlier when the blowdown is better.

Increasing CR will reduce blowdown time, but increasing VE will increase blowdown time. Blowdown for an NA gas engine is substantially different than for a blown nitro engine beyond just the differences in pressures when the exhaust valve starts to open.

For an NA gas or methanol engine blowdown should be done fairly early on the exhaust stroke to minimize pumping losses on the exhaust stroke. Blowdown (reduction in cylinder pressure to relatively near atmospheric) may only take from the time the exhaust valve starts to open until the piston is not too far up on the exhaust stroke. Of course the volume of exhaust remaining must come out, but this is accomplished with not very much increase in cylinder pressure assuming the exhaust is working properly.
....
Low power engines with poor exhaust scavenging designs may need excessively large exhaust valves simply because there is little or no suction in the exhaust port at any point in time. If the only thing getting the exhaust out is the piston, then the exhaust valve has to be big.

It is interesting to take a well tuned/cammed engine with a decent exhaust and dyno test it with little or no exhaust system attached. It gives you a real appreciation for how important the exhaust really is.

Pressure wave tuning, at least the way we define it, also takes into account port velocities. If you take two similar conventional exhaust systems in that both have the peak suction timed say for TDC valve overlap at some rpm, the exhaust with the smaller diameter (higher velocity) will usually have a little fatter powerband, to a point. Adding steps and various other things can fatten the powerband some more. Wave tuning and velocity are complimentary in that changing the port diameter and/or initial pipe diameter affects the shape of the wave, not just its location relative to valve overlap and its the shape of the wave (not the magnitude nor tuned rpm) that controls the powerband width. In general the wave shape is more important than the magnitude. Decent peak magnitude is not difficult to come by. The wave shape controls not only the powerband width, but the blowdown process as well which can lead to all kinds of possibilities as touched on by various people earlier in the thread.

Calvin Elston:

In my world, reversion is the biggest problem. I define reversion as pressure waves from the other cylinders in the system going the wrong way at the wrong time. They hurt a lot. Venturi collectors, 421 tube layouts, and controlling sudden area changes have all helped to control them. It is no longer beneficial to try and stop them in the primary tubes, and your observation is the reason, I think. A "reversion dam" also hinders a negative wave also, and that is not good!
I currently use a large step, ie-3 tube sizes, in some pro-stock headers I build, but it is not at the head surface, I move it out about 6 inches. They can work for some tuners but I see them as a way to assist their need for a rather large header.
ProStock,and subsequent benefactors,has been under a relatively steep learning curve over the last 4 years or so. Since Anderson tapped into to Cup technology, advances in ring seal, valve train/Cam etc, take any focus away from headers. The need is for a header big enough to not get in the way of the other advancements. This is good! Eventually they will be back into the headers searching for any more power thay may have left behind in the rush!

I think almost all my efforts over the last ten years with PS type engines is to increase the velocity and for a good % of my customers it has been a distinct advantage at times when others are simply making everything bigger. An exhaust system that is only helping with pressure wave tuning is identified not by a lack of horsepower but rather by a rather sudden drop in power after the peak. A exhaust system that also incorporates or tries to add velocity at least in the first third of the system is identified by at least the same power level but it has a much broader curve and carries much farther after the peak.
I have found over the years, and this is what I started this thread with, is that I can only reduce the area of a port I am given to build a header for by about 10-12% before the power falls off. So my conclusion is that maybe the ports themselves are too large and why can't we try smaller ports and go from there! It has been a frustrating journey

Cheers,
Bob

 

[scabber]

Yamaha put the crossover before the mufflers. It seems like a better place. I might try building a set with the 1 1/2" long radius ells. The ones I have on the bike now are a poor design comming off the head. I was thinking of going with 1 5/8 " after the antireversion chambers and step down to 1 1/2 at the muffler. Right now the bike with shell # 1 cam pulls hard to 6500 RPM and power drops off. My thinking is to extend the power band to 7000 or 7500 rpm with the larger pipe. Not knowing the rpm range the cam was the designed for I might be wasting my time. Or not just a thought. I do think it would not cause a power drop from what I have on the bike now.

 

[hoffman900]

Scabber,

Not sure how involved you're looking to go.

This is just a rough estimate for a 2-2, but if you're looking to go custom, maybe try something like this for a set-up like yours:

1.5"od for 15"
1.625"od for 15".

Total system length of 49". So, after 30" of primary pipe I would do either 1) megaphone over that 19" (+/- an inch or two) - which there should be an off the shelf item or 2) 1 3/4" pipe before entering the muffler (note this is not what you would think of a 3 step header).

A 3 step header would go like: 1.375 > 1.5 > 1.625 each 10" long. Of course, talking tubing this small, you need to make sure you're working on a 6" bend radius. 1.5" exists in that radius, but a 1.375" od pipe would have to be custom bent.

2-1:

1.5" for 30" (to the point of convergence in the collector) to a 2.5" pipe for 17".

At the very least, I wouldn't taper the exhaust back down. Crossovers are very dependent on location. A friend of mine was a friend of Mark Donohue and has a tape recording of them talking. He said they wore one of their AMC Trans Am engines out working on exhausts (1970)- a lot of the time spent on figuring out crossover location. People calculate their locations to get in the ballpark location, but to get them really to work, you need to get on a dyno and move them around to fine tune them.

If you still have the stock XS crank phasing, you would probably be better served building a 2-1 exhaust than one with a crossover.

That's my best guess of your set up. YMMV and all that. There is a lot of data for these bikes using larger set-ups which would be the safe route.

Also, make sure you come out to the AHRMA race at NJMP this summer. You''re only 35 minutes away from there.

 

[grizld1]

Scabber, a fatter pipe won't help with your issue, it will just move the torque peak farther into rpm's that you're not hitting as it is. Try retarding the camshaft by 2* first. Also, run a leakdown test; if you followed Joe Minton's advice and used stock springs with the Shell #1, you may have cupped a valve (and please don't ask how I know).

 

[Jack]

Yamaha put the crossover before the mufflers. It seems like a better place. I might try building a set with the 1 1/2" long radius ells. The ones I have on the bike now are a poor design comming off the head. I was thinking of going with 1 5/8 " after the antireversion chambers and step down to 1 1/2 at the muffler. Right now the bike with shell # 1 cam pulls hard to 6500 RPM and power drops off. My thinking is to extend the power band to 7000 or 7500 rpm with the larger pipe. Not knowing the rpm range the cam was the designed for I might be wasting my time. Or not just a thought. I do think it would not cause a power drop from what I have on the bike now.

have you reworked the exhaust ports? If not,this why your power drops off around 6500. The exhaust ports are to damn restrictive and will limit breathing higher up the RPM range

 

[scabber]

Scabber, a fatter pipe won't help with your issue, it will just move the torque peak farther into rpm's that you're not hitting as it is. Try retarding the camshaft by 2* first. Also, run a leakdown test; if you followed Joe Minton's advice and used stock springs with the Shell #1, you may have cupped a valve (and please don't ask how I know).

Not really sure I have an issue. From my seat of the of the pants dyno peak torque is around 6500 RPM. The bike not falling on it's face after that it still pulls the bike just not as fast. Not knowing the Rpm range shell designed the cam for doesn't help. I am not running stock springs. I know you were running the shell #1 in your 700 motor. How did work.? Run strong to redline 7500 ? Jack I did The ports in the head from your design the old garage site. I did not shorten the guide. I did feather it. Most of the work in the guide boss area valve pocket and the other areas you adressed. Mikes off the shelf 1 1/2'' Pipe leaves a lot to be desired to say the least. Hoffman Thanks for your advice on the exhaust. I might get over to the races this summer. I have it on my to do list now. My brother ran a set of stahl headers on his N stock 55 chevy back in the day. It held the NHRA record for a while. All things being equal a better designed exhaust will put you out front. It pays to think outside the box.

 

[Jack]

Scabber,the shell # 1 will pull up to 8000 RPMs maybe + if you have the head that's ample enough to supply the air. The protruding VG in the E/P is just one of the contributors of resistance and flow limiter and just feathering it doesn't really change anything. You need to look at the carburetors,headers or the heads.

 

[scabber]

Thanks for the information on the cam Jack and the guide. Nice to know. All in all for a street bike the low end power the cam puts out with the short falls of the package I'am running now. Restrictions of the stock air boxes and carbs fine tuning I haven't finished. I'am happy with it now. Nice to know will go to 8000 Rpm. Thanks again Jack.

 

[650performance]

Scabber, the Shell #1 has a nice, long power peak from 7000 - 8000 and can overrev to about 8300 with only a minor fall off. If your power is hitting its peak much lower than that then there are obviously problems with moving enough air. Whether that is carbs (too small), head (not enough air flow), or exhaust (too much back pressure), or a combination of all three is a whole new thread.

 

[TwoJugs]

I get lots of enjoyment reading all this tech stuff, sometimes 3 times to get a grip on what is being said!

I did see some mention on a 2 into 1 exhaust. Anyone using one in a built motor or is there a reason to stay away? I am working on a 750 motor build now based off some items I have from a buy a few years ago. I plan on running a Shell #3 cam. Just had the crank welded up after a set of new Shell rods were installed. The cylnders are out now getting sleeved/bored for the Shell 400 slipper pistons I had sitting in the boxes.

I did some mild porting, mostly cleaning up the casting work and smoothing out some of the radius areas. I wanted to open up the exhaust a bunch seeing that the holes appeared very small but now I move with caution. I do have a bandsaw cut away of a head showing very little room to grind in certain areas. I will post some phots of it soon. Very interesting to see what is going on behind the metal. I have 1 3/4 Shell pipes that I plan on using. Makes me nuts looking at that big ramp they left in the port before it will get to the exhaust gasket and pipe. The motor will be turning up some good revs so not sure if I should leave it alone or open it up. I already ran the bike with a mostly stock motor. looking to compare as I go along with the changes but mostly trying not to break anything that is going to cost $ that I don't have to throw at it right now. I am almost ready to set up the cam and check spring clearances. Sometimes I want to just stop reading and slap it together to see what it does. Other days I feel the need to read, research and go radical.

 

[hoffman900]

Twojugs,

2-1 is dependent on firing order.

Here are some recent posts by Calvin Elston who I've referenced earlier:

In a given engine at a given rpm on the exhaust side I want to keep the gases moving in one main direction, out. Nice

This involves keeping the diameters of tubing and area changes as small as possible in order to keep the velocities of gases up on their way to atmosphere. Hard

Unlike on the intake side; If you introduce a sudden area enlargement to a flow of exhaust gases, they will slow down because of it and you can not re-accelerate them back as you have traded velocity for pressure and heat.
This is why I am so particular about matching port areas with tubing off the head as the energy is greatest here and wasting it here hurts more than wasting it at the other end.

The higher the velocities the harder it is for them to turn around and go the wrong way. Good

The further the "slug" or "mass" of exhaust molecules can get from a individual cylinder on a individual exhaust event the further they have to travel back to the cylinder at times of "reversion" or valve overlap. Good

Reversion is always happening, the amplitude and its ability to move exhaust flow direction are what we are controlling with both the header and rpm (time for it to happen) the trick.

The higher the velocities the less the pressure locally and for that matter the entire exhaust header. Good

The larger the diameter of the hole at the end of a header system, the harder it is to maintain less pressure in front of it (in the header) compared to atmosphere. My reality.

Smaller diameter tubing offers more flow resistance, bad.
Tighter radius bends offer more flow resistance. Ever tried it on a flow bench? Bad

To design a exhaust header to help a engine make as broad a power curve as possible, we are balancing all the above.

How small can you keep the tubing diameter for how long, and how small a final hole can you get it all through and try to protect whatever depression you have generated. The game.

Notice I never used the word "tuning" anywhere above. That should get a a few responses.

That is why we step headers. Ther is no reason to use a tube larger than area of port. If the port is big enough to make good power...do you need to help it with a bigger header? Match the port area to the nearest tube size and step from there, max of 10-12" per step infill you get to desired length. This is not complicated. The main issue is doing a smooth transition from port to header and be as smooth as you know how

Thought I would point out another interesting reality seldom thought about.
The percent area change at a normal/common header step of .125" diameter change.
It is a much larger percent change when down around 1.5 or 1.625 tubing like a SB chev at say 500hp verses 2.12-2.25" like a BB chev.

Stepping at smaller diameters is a much bigger deal and impact velocities much more than larger tubes on bigger engines.

Another comment in the discussion from another engine builder:

Working on bike engines I can definitely second this. Stepping in .125" increments means you need to make sure the steps are correctly located. Even small changes in wall thickness, or using mild steel vs stainless vs titanium, or differences in airflow over different primaries, can change things noticeably.

A couple very successful head porters (like high end V8 stuff), one that has personally told a friend of mine, is they don't worry about exhaust port cfm. Just having the right velocity and a quiet port.

I might build a new head pipe eventually on my bike with a 6" radius bends keeping all the above in mind. There are a few sources for those bends in smaller diameters.

Keep in mind, high end V8 racing headers are using 1.5" off the head on 500+hp SBCs and SBFs. That's 100hp a L, or the equivalent of a XS650 displacing 750cc making 75hp...

 

[Tomterrific]

Exhausting mistake.

 

[hoffman900]

If anyone is curious about this stuff, there are two parallel threads on Speedtalk in the advanced section (have to register (it's free) to view it). One is called 'Two Cylinder Exhaust Design' and the other is an offshoot of it called 'Exhaust Design - Exhaust Pressure Traces'.

 

[scabber]

Thanks for the links. I changed the exhaust on my bike to a modified commando system. I shortened the 1 1/2 head pipes to 35 1/2" and installed the commando mufflers. The carb jetting was wrong but could tell top end power improved. Never got the BS 34 carbs to work at optimum performance. I scrapped the BS carbs and went with the PVK carbs of the EX 500 Kawa Uni pod filters. With the new carbs it is a whole new bike. Top end is unreal 5000 RPM to 8000 the exhaust and carbs are flowing there ass off. It is like a 90 MPH sweet spot to 120 top. The exhaust sound changes and the motor runs smoother and faster less vibes. I really think the CVK carbs flow better and meter fuel better than the BS carb. The reason for the less vibes to my way of thinking. I'm shure the exhaust also a factor.